1. Field of the Invention
The present invention relates generally to channel estimation in a transmission diversity system and, more particularly, to a method for channel estimation/antenna verification in WCDMA (wideband code division multiple access) closed loop mode 1 transmission diversity.
2. History of Related Art
To be able to use channel estimates from the usually strong common pilot channel (CPICH), the phase difference between CPICH and the dedicated physical channel (DPCH) on the diversity antenna needs to be estimated. This is, in the Third Generation Partnership Project (3GPP) documents, referred to as antenna verification. Some methods for antenna verification are shown in the 3GPP documents [1] 3rd Generation Partnership Project (3GPP), 3G TS 25.101 V3.4.0, October 2000 and [2] 3rd Generation Partnership Project (3GPP), 3G TS 25.214 V3.4.0, September 2000. If DPCH is used for channel estimation, the implementation margin is only 1.2 dB.
In WCDMA base station transmission diversity can be used. Four methods for operation are specified by 3GPP:
1. No TX-diversity
2. Open loop TX-diversity
3. Closed loop TX-diversity mode 1
4. Closed loop TX-diversity mode 2
In Closed loop mode 1, different antenna 2 settings are used for CPICH and DPCH. This means that the channel estimates from CPICH cannot directly be used for DPCH. An advantage of using channel estimates from CPICH is that the channel estimates from CPICH usually are less noisy than channel estimates from DPCH, due to the high transmission power for CPICH. The fact that the DPCH does not have the same transmission conditions as the CPICH makes it impossible to directly use the CPICH for channel estimation. The estimation of the applied phase shift is also known as antenna verification.
Antenna settings for CPICH and DPCH can only differ in known ways. Depending on feedback information (FBI) commands sent from a user equipment (UE) to a base station (BS), an extra phase advance φ∈{π/4,3π/4,5π/4,7π/4} is added to antenna 2. Using information about the channel estimates for antenna 2 and the generated FBI commands, the phase advance φ can be estimated. In closed loop mode 1, the user equipment (UE) instructs the base station (BS) how to apply a phase shift for the dedicated physical channel (DPCH) on antenna 2 (i.e., the diversity antenna) with FBI commands on the uplink. This phase shift is one of {π/4,3π/4,5π/4,7π/4}. The phase shift makes the common pilot channel (CPICH) and DPCH have different channel coefficients for antenna 2.
If there were no transmission errors on the feedback information (FBI) commands on the uplink, the UE would directly know the phase shift that is applied by the base station. In the presence of errors on the FBI commands the UE can, however, improve the situation by observing the phase difference on second (diversity) antenna channel estimates from CPICH and DPCH.
One method for antenna verification is described in Annex A of [2] 3rd Generation Partnership Project (3GPP), 3G TS 25.214 V3.4.0, September 2000 (hereinafter [2]). In this method, a certain inequality is checked after each slot. When the UE determines if a phase shift in the right or left half plane is used, the following inequality is checked.
      2    ⁢                  ∑                  i          =          1                          n          fingers                    ⁢                                    2                                σ            i            2                          ⁢                  ℜ          ⁡                      (                          γ              ⁢                                                          ⁢                                                h                  ^                                                  2                  ,                  i                                D                            ⁢                                                h                  ^                                                  2                  ,                  i                                                  C                  *                                                      )                                >      ln    ⁡          (                        Prob          ⁡                      (                                          π                /                2                            ≤              ϕ              <                              3                ⁢                                  π                  /                  2                                                      )                                    Prob          ⁡                      (                                                            -                  π                                /                2                            ≤              ϕ              <                              π                /                2                                      )                              )      where nfingers is the number of fingers used in the RAKE combiner, σi2 is the variance of the Gauss process, h2,i^D is the channel estimate for the second antenna, ith RAKE finger and DPCH, and h2,i^C* is the complex conjugate of the channel estimate for the second antenna, ith RAKE finger and CPICH.
If the inequality holds, it is estimated that −π/2≦φ<π/2. In the above inequality, γ2 is the DPCH signal-to-noise ratio (SNR)/CPICH SNR. The multiplication with γ will scale the above to an SNR-like expression for DPCH. The probabilities are evaluated depending on sent FBI commands. If upper or lower half plane is being decided, the following inequality is checked:
            -      2        ⁢                  ∑                  i          =          1                          n          fingers                    ⁢                                    2                                σ            i            2                          ⁢                  𝔍          ⁡                      (                          γ              ⁢                                                          ⁢                                                h                  ^                                                  2                  ,                  i                                D                            ⁢                                                h                  ^                                                  2                  ,                  i                                                  C                  *                                                      )                                >      ln    ⁡          (                        Prob          ⁡                      (                          π              ≤              ϕ              <                              2                ⁢                π                                      )                                    Prob          ⁡                      (                          0              ≤              ϕ              <              π                        )                              )      
If the inequality holds, it is estimated that π≦φ<2π.